2 Answers
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To achieve this assign the public ip to host system. Then forward the required port from host system to guest system.

Suppose your guest is a web server then you need to forward port 80 from host system to port 80 of guest system so that every request for port 80 on public ip will be forwarded to port 80 of the guest.

Suppose you have assigned public ip to eth0 and your guest's ip is 192.168.1.2, then you have to tell host that it should do NAT/MASQUERADE so that the virtual machines have internet access. You also have to tell host which ports it should forward to which IP address. You can refer following script.

The first two commands enable Nat/Masquerad'ing on host. In the section after ### Port Forwarding ### you put as many rules as you need. This tells host to forward certain ports to certain destination ports on certain destination IP addresses.

You should think about the virtualized machine as about real machine. You shouldn't have two machines with the same IP in your network. But you may use some bridge, NAT, port forwarding, if your goal is to hide the presence of virtualized environment.